1. Field of the Invention
The present invention is directed to an apparatus for measuring the properties of an emitted laser beam, and more particularly to an apparatus for determining the characteristics of an optical fiber in the laser infrared wavelength range.
2. Description of the Prior Art
Fiber optics have become an important electro optical component in numerous information and communication systems. The ability of a specific optical fiber to carry energy is an important design consideration. The current trend in the optical fiber communication field is to develop optical fibers and optical components for the so-called second and third fiber transmission windows at 1.3 and 1.6 microns, respectively. It has been discovered that these transmission windows are characterized by low optical fiber energy losses due to reduced Rayleigh scattering which exhibits .lambda..sup.-4 wavelength dependence. The increased recognition of the advantages of this technology has created a requirement to measure and quantify the characteristics of these optical fibers. One of the conventional forms of a measurement standard is the numerical aperture which is the sine of the half angle of the widest bundle of energy transmission. This measurement of an optical fiber determines the light gathering ability of the fiber, that is, it defines the half angle .theta./2 of the light acceptance cone. Light waves injected at angles within this cone will be waveguided, while rays entering the fiber core at steeper angles will be lost. The numerical aperture, NA, is related to the acceptance cone angle by the following relation: EQU NA=n SIN .theta./2=.sqroot.n.sub.1.sup.2 -n.sub.2.sup.2
wherein n is the index refraction of the interfacing medium, such as air, n.sub.1 and n.sub.2 are the refractive indices of the core and cladding materials respectively of the optical fiber. Thus, a measurement of the numerical aperture is an important parameter in determining the coupling efficiency of an optical fiber to a source, such as an LED or laser. It is also important in calculating the injection losses when dissimilar fibers are connected and in determining the susceptibility of a fiber to microbending.
The standard procedure for measuring the optical fiber numerical aperture usually follows the guidelines issued by the National Bureau of Standards in cooperation with the Electronic Industries Association and calls for measuring the accuracy to within two percent. The traditional techniques for measuring the numerical aperture of optical fibers comprise an expensive and complicated interferometric technique, which measures the optical fiber refractive index profile from which the numerical aperture can be calculated, or alternatively, a relative simple technique which measures the far field pattern of radiation emerging from the optical fiber. Both of these techniques, however, use visible light and consequently the experimental values of the numerical aperture obtained can be as much as ten percent different from the true values of the numerical aperture experienced for the transmission of infrared wavelengths.
Utilizing conventional equipment has proved difficult in measuring infrared transmission in optical fibers. Conventional infrared sources are injection laser diodes and incandescent sources which can be expensive, and in the case of laser diodes emit low power, while in the case of the incandescent sources can be cumbersome. There are no practical LED's that emit in the infrared region and there is a farther problem in imaging the infrared radiation, since silicon diode matrix arrays are relatively insensitive to radiation wavelengths longer than one micron, and pyroelectric detector matrix arrays are very expensive and suffer from inadequate resolution.
Thus, there is a demand in the prior art to provide a relatively inexpensive, high resolution apparatus for measuring the numerical aperture of infrared radiation transmission in optical fibers.
Additionally, there is a demand to provide an improvement in the measurement of the total energy, relative position and divergence of an infrared laser beam over the prior art disclosed in U.S. Pat. No. 4,320,462.